In modern aircraft engines, it is essential to monitor precisely the mass flow rate of fuel supplied to the engines. One general type of motorless in-line flowmeter for aircraft is shown in the following U.S. patents:
______________________________________ Patentee U.S. Pat. No. Issue Date ______________________________________ Vetsch 3,877,304 April 15, 1975 Baker et al. 3,958,447 May 25, 1976 Chiles et al. 4,012,957 March 22, 1977 Cheney, Jr. 4,438,648 March 27, 1984 ______________________________________
In a motorless in-line mass flowmeter of the general type shown in the above patents, fuel first passes by and imparts rotary motion to a turbine mounted on a longitudinally extending shaft. The fuel then passes through a stationary component having passages for "straightening" the fuel so as to remove any circumferential moment and direct the fuel axially toward the next component of the flowmeter. The rotating shaft passes through the straightener and then through an impeller having radial vanes in the path of the fuel. The impeller is mechanically interconnected with the shaft by a torsion spring such that torque is imparted to the impeller by rotation of the shaft. Depending on the mass of fuel passing through the flowmeter, the impeller will lag behind the shaft due to the drag of the fuel passing between the radial vanes of the impeller. Usually the impeller is received in a drum fixed to the shaft and the torsion spring is connected between the drum and the impeller.
In the known flowmeter, the drum, the impeller and adjacent mechanical components, including the casing of the flowmeter, are formed of nonmagnetic material such as an appropriate aluminum alloy; but both the impeller and the drum carry small magnets. Electromagnetic pickup coils are used to detect rotation of the magnets. Usually the pickup coils are mounted externally of the flowmeter, although in some constructions the pickup coils are internal. The coils can be aligned radially with the circular paths travelled by the magnets, or the coils can be aligned axially with the magnets. Low level inductive pulses are generated which are analyzed to determine the speed and deflection angle of the impeller. The mass flow rate of fuel passing through the flowmeter can be calculated from the deflection angle and speed. Care must be taken to assure that electromagnetic interference does not impede performance and accuracy.